In recent years, since a micro electro mechanical system (MEMS) based sensor technique, a low-power analog and digital electronic technique, a low-power RF design technique, and the like have been rapidly developed, a network technique using comparatively low-cost, low-power type wireless sensors have been actively developed.
Since self-construction in wireless sensor networks (hereinafter, referred to as WSN) as a very important technique for construction of a large-scale network comprising a large number of sub systems can be generally used for improving network extensibility and dynamically adapting to a change in environmental condition, it plays a very important role in a communication system.
Cluster-based self-construction in the WSN segments an entire network by the cluster unit to present a networkbased structure for easy control and uses a hierarchical approach that divides an entire WSN into cluster classes, and respective sensor nodes form one cluster around one cluster head and segment the entire network by the cluster unit. A cluster head of each cluster serves to route data which are acquired, collected, and merged in each cluster, to a cluster head of another cluster or a base station.
The process of self-construction of the WSN generally comprises three steps, that is, a network structure determination step of determining the construction of the network, a network connection step of connecting devices and extending the network according to the determined network structure, and a data transmission step in which the connected devices transmit and receive signals to and from each other. When the devices of the WSN access to the network, they are granted with their own unique addresses to recognize each other by the address, data transmission and reception between intra-cluster devices comprising one parent device and a plurality of child devices are performed within the same superframe duration, and devices which are far from each other transmit and receive data through multi-hops due to limitation of a transmission distance of the device. Therefore, a unique address is allocated to each device for self-construction of a large-scale WSN, the superframe duration used by each cluster is independently allocated in order to prevent cluster-to-cluster interference, and it is essential to search for a routing path for multi-hop transmission.
A balanced-tree based WSN self-construction process of Zigbee which is a representative WSN system can be described in what follows. First, the network structure determination step makes a coordinator that controls the entire network construct a balanced-tree based network according to a maximum network depth, the maximum number of child devices, the maximum number of child routers, and the like. Thereafter, in the network connection step, each device determines all routers within a communication range as a parent candidate group, and selects a router in which a network depth is smallest among the routers as a parent, and as a result, a full function device requests subscription as a router and a reduced function device requests subscription as an end device. The router assigned as the parent determines whether to accept the device that requests the subscription by itself in consideration of the maximum number of child devices and the maximum number of child routers and thereafter, directly allocates addresses to the devices exploiting a balanced-tree structure, establishing the parent and child relationship. In this case, to determine the balanced-tree structure, each parent device transmits a beacon signal containing parameters including the maximum network depth, the maximum number of child devices, the maximum number of child routers, allowing the child devices to share the balanced-tree structure.
When a specific device cannot make a subscription to any of all neighboring routers, it recognizes network subscription failure and becomes an orphan device, and when the full function device cannot make subscription to the network as a router, it re-tries to make subscription to the network as an end device.
Finally, devices connected to the network can transmit data by means of hierarchical routing associated with the balanced-tree structure in the data transmission step.
Zigbee can make self-construction of a balanced-tree based WSN while providing advantages of using reduced time and traffic for exchange of control command messages in the network construction, and requiring no additional search for routing path in a tree based hierarchical routing, but it may have problems in that the network depth is limited due to a characteristic of a balanced-tree in which the size of the tree structure is exponentially increased according to the network depth and the number of useless addresses is increased as the formed WSN is enlarged because the balanced-tree structure is determined without considering operating environments. Further, since the full function device that can the router or the end device requests subscription as a router until subscription of the full function device is rejected by all neighboring parent candidates, the routers are concentrated at the vicinity of the coordinator, and as a result, it is difficult to extend the network far even though the network depth is increased. Moreover, since a channel is saturated and thus the clusters cannot have an independent superframe when the density of devices is high due to a constraint condition in which an entire network uses a single channel, connectivity of the network is significantly degraded due to cluster-to-cluster interference.